There has been developed a transparent screen that uses, as one convex lens, a Fresnel lens including a number of concentric circular members and that focuses a beam of light emitted out of the Fresnel lens onto an image formation/display plate so as to obtain an image. For example, international patent publication No. WO02/27399 discloses a transparent screen provided with a refraction/total reflection plate (i.e., a Fresnel lens) which has a portion for refracting a ray of light to be projected and another portion for totally reflecting the ray of light to be projected, and an image formation/display plate for focusing a ray of light emitted out of the refraction total reflection plate so as to obtain a projected image.
A refraction/total reflection plates disclosed by international patent publication No. WO02/27399 has a number of slanting parts formed on a surface thereof upon which the ray of light to be projected is incident. Those slanting parts include refraction slanting parts each for refracting an incident ray of light to be projected so as to make it travel towards the image formation/display plate. Those slanting parts also include total reflection slanting parts each for totally reflecting a ray of light to be projected passing through a transmission slanting part located just below the total reflection slanting part and entering the inside of the Fresnel lens, and for then making the ray of light to be projected travel towards the image formation/display plate. In other words, each total reflection slanting part reflects the incident ray of light to be projected, which travels the inside of the Fresnel lens, toward the inside of the Fresnel lens. In the Fresnel lens, scattering particles which exhibit weak dispersion characteristics are dispersedly disposed, and the viewing angle of the display image light is mainly determined by a combination of the dispersion characteristics and dispersion characteristics which the image formation/display plate exhibits.
“Shikama, S. et al., Optical System of Ultra-Thin Rear Projector Equipped with Refractive-Reflective Projection Optics, SID2002 Digest, 46.2, (2002)” discloses a projection display apparatus using such a prior art type of transparent screen. Since this reference is thus cited in this specification, it can be assumed that the written contents of this reference make a part of disclosure of this patent application.
The inventors of the present application found out by both experiment and light ray tracing simulation that obstruction light was visually recognized in addition to effective beams of light which contribute to a regular projected image display in the transparent screen using the above-mentioned refraction/total reflection plate. Therefore, the transparent screen has been in need of improvements in the obstruction light occurrence phenomena in order to produce a high-definition image display. For example, although in each of the total reflection slanting parts, most of a ray of light to be projected incident upon a corresponding transmission slanting part should pass through the transmission slanting part and then enter the inside of the Fresnel lens, a part of the incident ray of light is reflected by the transmission slanting part and then becomes a downward ghost ray of light after passing through an unexpected path, so that a watcher can visually identify the downward ghost ray of light. In addition, while each of the refraction slanting parts refracts an incident ray of light to be projected and makes it travel towards the image formation/display plate, a part of the incident ray of light to be projected can enter an ineffective facet part that is located directly under and is adjacent to the refraction slating surface portion and can become an upward ghost ray of light or double image light after passing through an unexpected path, so that a watcher can visually identify the upward ghost ray of light or double image light.